This invention relates to an acceleration detector and more particularly to an acceleration detector for detecting knocking in an internal combustion engine.
FIGS. 1 and 2 illustrate one example of a conventional acceleration detector. The acceleration detector comprises a housing 1 defining an annular cavity 2 therein and an annular acceleration transducer assembly 3 disposed within the cavity 2. The housing 1 comprises a tubular, electrically conductive metallic bushing 4 having a through hole 5 and a flange 6. The housing 1 also comprises a ring-shaped risinous outer case 7 bonded by a bonding agent 7a to the flange 6 of the bushing 4 so that the cavity 2 is defined therein.
The outer case 7 also has a connector 8 radially outwardly extending from the outer case 7 so that an external output terminal 9 can extend through the connector 8 for taking out an output signal from the acceleration transducer assembly 3 disposed within the cavity 2. The acceleration transducer assembly 3 further includes an annular piezoelectric element 11 placed on the terminal plate 10, a washer-shaped output terminal 12 including a lead 12a connected to the external output terminal 9, an electrically insulating washer 13 disposed on the output terminal 12, an annular inertial weight 14 placed on the insulating washer 13 and a threaded ring-shaped stop nut 15 thread-engaged with the thread 4a on the tubular bushing 4. An electrically insulating tape or tube 16 is placed on the tubular bushing 4 so that the acceleration transducer assembly 3 is insulated from the bushing 4 even when the output terminal 12 as well as the piezoelectric element 11 are eccentrically assembled.
In order to resiliently support and protect the acceleration transducer assembly 3 within the cavity 2 from undesirable environmental conditions, the remaining space of the cavity 2 of the housing 1 which is not occupied by the acceleration transducer assembly 3 is substantially filled with a resilient filler material 17 of a thermo-setting resin. The filler material 17 must be sufficiently resilient after it is cured to allow the movement of the inertial weight 14 relative to the housing 1 when an acceleration is applied to the inertial weight 14 so that the piezoelectric element 11 generates a voltage signal proportional to the pressure exterted on it by the relative movement of the inertial weight 14 against the piezoelectric element 11.
When in use, the acceleration detector is securely mounted on the internal combustion engine (not shown) by a suitable bolt (not shown) inserted into the central through hole 5 of the housing 1. The acceleration or the vibration of the internal combustion engine produces the movement of the inertial weight 14 relative to the housing 1, which causes the piezoelectric element 11 to be stressed by the inertial weight 14, whereby an electrical signal indicative of the movement of the inertial weight 14 relative to the engine is generated from the piezoelectric element 11. The electrical signal is provided from the output terminal 9 analyzed to determine whether knocking of the internal combustion engine is involved. One example of the output from the output terminal 9 is illustrated by a curve A in FIG. 3. When it is determined that a knocking signal is contained in the electrical signal, the operating parameters for operating the engine can be adjusted to increase the output power or decrease the fuel consumption rate.
In the conventional acceleration detector as above described, the output signal indicative of the acceleration supplied from the acceleration detector varies relatively significantly according to the frequency of the oscillation as shown by the curve A in FIG. 3 in which output voltage is plotted against the oscillation frequency with a constant magnitude of the oscillation. As seen from the curve A, the output voltage gradually increases as the oscillation frequency increases over a wide oscillation frequency range of the acceleration to be detected and the slope of the curve A is particularly steep in a higher frequency range of more than about 10 kHz. Moreover, there is an abrupt change with a high peak and a deep valley at the oscillation frequency around the frequency range well above 10 kHz. This is undesirable because the detection characteristics of the acceleration transducer assembly 3 is disturbed and the output of the detector does not correctly indicate the magnitude of the acceleration.